The Great Unknown
by SupernaturalWhoLocked
Summary: Sherlock Holmes has always know that sentiment is found on the weaker side, but when Moriarty has come back and has started an uncontrollable fire, Sherlock learns that sentiment can be found on the winning side if you can control the flame that is love and friendship and family...But who do you have to lose to win? *New Character*Holmes Family feels*


Chapter One: The Government, The Detective and the Sister

 **A/N: This is my first time writing for Sherlock and I can't wait to write! Sherlock is such an interesting character and his character development throughout the show is amazing! This is a post season 3 story and so I sincerely hope you'll enjoy :) All cannon ships apply to this story as well.**

 **Now Mycroft said something interesting in the season 3 finale. He said "you know what happened to the other one" now I think there was another sibling of Mycroft's and Sherlock's. I think that's why they say that caring isn't an advantage and sentiment is a chemical found on the losing side. I think they had a sibling (whether a brother or sister, I'm not sure) and the sibling died. Just my theory but it works out I think.**

 **So I'm going with sister in this story. Her name is Cassiopeia Elizabeth Holmes, she has bright blue eyes and black hair. I imagine Lana Parrilla (with blue eyes of course) playing Cassiopeia Holmes. We know that Sherlock and Mycroft have these great deducing minds, however I imagine Cassie having this low level of deducing "powers" however she understands humans better than her brothers. She's straight forward however she has a better way of saying it than Sherlock and Mycroft.**

 **So onto the story!**

 **Disclaimer: I don't own anything you recognise :)**

***TGU***

' _Get out!' She screamed. They stood in shocked silence as the shorter girl stood up to them. To them all. However she was sick of them. Of all of them. They didn't care about her. They were only using her. They didn't care about how she felt. None of them cared about her breakup with Tom. How she felt after that. They didn't ask why._

 _Or that's what she was told to think anyway._

' _Molly-' Sherlock said, taking a step forward but the girl with shoulder length black hair grab his arm and pulled him back._

' _I want every single one of you out of here.' Molly said. 'You don't care about me. You're all using me for some sick game. This- this game of yours Sherlock. It's not on anymore. It's over. If you lot want to look at this body go to Mike and talk to him. Do the official paperwork.'_

 _Lestrade, John, Mary, Mycroft, Cassiopeia and Sherlock looked at her in shock. Molly did not care however. She was sick of them. All of them._

' _Molly-' Mary said, 'let's go talk. The café down the road-'_

' _Out.' Molly said simply, crossing her arms and clenching her jaw._

' _William Holmes, get back here,' Cassiopeia snarled grabbing Sherlock's arm. He had tried to move forward and he was pulled back by his older sister. Sherlock glared at her, however he did not try to move forward, for which Molly was internally grateful for._

 _Molly turned around and walked over to her phone._

Great job girlie. Convincing. Very Convincing. –TJ

 _Molly closed her eyes and turned back around._

' _What did I say?' Molly asked. 'Get. Out. Go back to Baker Street.'_

 _In that moment she saw Cassiopeia's eyes light up. Molly nodded so very slightly and she saw Mary glance at her phone. However the four men had not caught on. Considering they all worked in solving crimes together, it was quite surprising._

' _Let's go.' Cassiopeia said. Sherlock was about to protest but Cassiopeia just dragged her two brothers out of the room. John and Lestrade left and Mary followed them. Molly quickly walked over to her desk and looked at the photograph._

 _It was taken at Christmas. After Sherlock's name had been cleared from killing Magnussen. They were at Baker Street and they were all wearing Christmas jumpers (expect Mycroft and Sherlock) and they were all happy (Mycroft and Sherlock content) and Ginevra Watson, who was only a month old, was in Molly's arms._

 _Lestrade, Sherlock, John and Mycroft were standing up and Molly, Mrs Hudson, Mary and Cassiopeia were sitting down. Molly couldn't help but smile at the memory of the trouble Mary went through to get the camera timer set and then get everyone to smile (or in the Holmes brother's case, not look bored or annoyed at the sentiment)._

 _Molly looked at her phone again._

Don't forget our deal Miss Hooper. –TJ

 _She stared at her phone horrified. It was really happening. Someone was threatening her loved one and she had to do as they say. Expect she was being threatened by her ex-fiancé and the world's only consulting criminal and her loved one was the world's only consulting detective._

 _The game, Sherlock would say, was on._

***TGU***

 _ **One year earlier**_

Sherlock rolled his eyes as he saw Moriarty appear on the television screen of the car. This was obviously ruse from her. Moriarty was dead. Sherlock had watched him shoot himself on the top of Bart's hospital. But then again, he was meant to be dead as well. Perhaps someone was being used by Moriarty. Or threatened.

'We need someone to look at this footage,' Sherlock said, glancing at his brother.

'I just sent her a text and she said to meet her at her place.' Mycroft said. Sherlock nodded.

'Am I missing something?' John asked. 'Who is "her"?'

'A person who can help us with this problem.' Sherlock said simply. 'Her identity isn't really important but if you must know her name is Cassiopeia.'

'And her last name?' Mary asked. 'I mean last names always do help but then again I don't think there are many Cassiopeia's in the world.'

'Holmes.' Mycroft said. 'Cassiopeia Holmes. Now Sherlock, was it actually Moriarty on the roof top?'

'Of course it was,' Sherlock scoffed but doubt had been planted in his mind. At the time he hadn't thought much of it but now as he looked back on it, his mannerisms where different from usual. If he even had a usual.

'Holmes?' John asked. 'Is she cousin?'

'Of course not,' Sherlock scoffed, 'she's our sister, but I hardly think that matters at the moment. We have a problem. I don't think it was Moriarty on the roof top, much like I don't think it was Moriarty dating Kitty Riley. It was his twin.'

'You have a sister and you've never bloody mentioned her?' John asked. Sherlock rolled his eyes at John's slow thinking. Sherlock hardly thought that his biological connection to the woman that could help them defeat Moriarty was worth mentioning but John was strange like that.

'She was estranged for many years.' Mycroft said thoughtfully.

'Why?' Mary asked.

'She faked her own death so that her only brothers would be safe.' Sherlock said. 'She was being threatened due to her involvement in a high government case. She faked her death, not even telling us or our parents and she revealed herself to us just before I made myself known to the world.'

'I'm surprised Mycroft hasn't faked his death yet.' John said sarcastically.

'It could still happen,' Mycroft said.

'Like you're smart enough to fake your own death Mycroft.' Sherlock scoffed.

'How can she help us?' John asked quickly before the two brothers started to fight.

'She is an "expert" in human behaviour.' Sherlock answered shortly. 'She does deduce sometimes but she's never been as good as I. Or Mycroft.'

'What is her job then?' John asked. 'Mycroft finds the case, Sherlock solves the case and then your sister finds out why they did it? But isn't what you do?'

'People like to know why murderers kill,' Sherlock said, 'what makes a murderer, per say. She finds out. It's not usually as simple as the wife or husband cheated and in anger they "accidently" kill them. It builds up in the person's mind over many years before boiling over.'

'Huh,' Mary said, looking curiously at Sherlock then at Mycroft. 'So I guess you two love her then?'

'Love?' Sherlock and Mycroft scoffed.

'Caring isn't an advantage.' Mycroft said.

'And you two know that how?' Mary asked. Both men fell silent and the car ride to Cassiopeia's home was spent in silence. Sherlock hoped that Mrs Hudson, Lestrade and Molly were safe, especially because he didn't know if this video was true.

They arrived at Cassiopeia Holmes's country home about an hour later and Sherlock saw Mary and John take in the enormity of the house however Sherlock and Mycroft kept walking. They weren't impressed by the house.

They entered the house and John and Mary followed them. They walked through the hallway down to the living room. Cassiopeia Holmes was sitting in the armchair next to the fire, a glass of red wine in her hand and she was staring at the telly, which was playing Moriarty's message.

'About time you two idiots visited me,' Cassiopeia said, not looking at them, 'I thought you two had forgotten your little sister.'

'I am famous now,' Sherlock said sarcastically, moving towards her. She took a sip of her wine and her eyes narrowed at the telly and she paused it. She frowned and Sherlock turned to face the telly.

'It's him.' Cassiopeia said simply. 'Whoever you saw on the rooftop Sherlock wasn't Moriarty. This is Moriarty. I don't understand his game through.'

'I do.' Sherlock said bitterly. 'He wants you to go out of hiding. Another weakness for me.'

'No one knows I'm alive expect you two idiots and our parents.' Cassiopeia said. 'How could he know?'

'Magnussen.' Mycroft said. 'He knows a person's weakness. Their pressure point as he use to nicely put it. He would have known you're alive and he would have made a deal with Moriarty.'

'That person,' Mary said suddenly, walking up to the telly. She was looking in the bottom left hand corner where half a face was sticking out. Sherlock walked up and narrowed his eyes.

'Who is this person?' Mary asked, looking at Cassiopeia.

'It's Moriarty's wingman, Sebastian Moran.' Cassiopeia said.

'Moran isn't just Moriarty's wingman.' John said. 'That's Molly's fiancé.'

'Ex-fiancé.' Mary and Sherlock muttered. John smirked at them.

'Interesting,' Cassiopeia said. 'Who is Molly, Sherlock?'

'My pathologist.' Sherlock said. 'And just – just a friend.'

'Your pathologist?' Cassiopeia smirked. 'Mycroft, pay up.'

Mycroft rolled his eyes but he grabbed a five pound note from his wallet and handed it to their sister, who still not looked at them.

'Should we be worried for Molly's safety then?' John asked. 'That's her ex-fiancé and her ex-boyfriend. They'd both have personal vendettas against her. It doesn't help that she's involved with Sherlock. She's a weakness of his.'

'She is not,' Sherlock lied sharply, 'she's simply my pathologist.'

'You don't use possessive words over someone who is simply a pathologist, Sherlock,' Mycroft said.

Cassiopeia stood up abruptly when they heard planes flying over the house.

'Get out!' Cassiopeia yelled. 'This house is about to blow up!'

Sherlock quickly pushed Mary and John in front of him and Mycroft was the last person to leave the house when the bomb hit the house. Mycroft and Sherlock were knocked forward from the force of the blast. Cassiopeia had quickly grabbed them and they ran towards the car.

'Have fun Sherlock!' A voice called out. He looked behind and saw Moriarty's face plastered on the side of a plane. 'You better get her before I do.'

Sherlock stared at the plane as it flew off into the sun and he glanced at Cassiopeia's burning house. His sister stood in horror as her whole life was burnt in the orange flames that represented so much more than just a burning house.

Their lives were just about to go up into flames.

'We have to go.' Cassiopeia said quickly. 'It's not safe for us.'

They quickly got into the car and John drove the car and Mary sat in the front, leaving the three siblings to sit together in the backseat. Sherlock saw Cassiopeia look at her burning house- home- through the rear view mirror and it took quite a bit of control to not scoff or roll his eyes. Sentimental. She was always so sentimental. It was only a place where she lived.

'No it's not Sherlock,' Cassiopeia said quietly. Sherlock looked at the youngest Holmes in mild surprise. 'It's not just a house Sherlock. It's a home. I've spent almost all of my adult years there. Never leaving unless absolutely necessary. It's like I've lost apart of myself. I don't understand how you, how _both of you,_ have lost all respect for human race.'

'Caring is not an advantage, sister,' Mycroft said, 'surely you know that.'

'Don't you dare, Mycroft Harold Holmes,' Cassiopeia spat. Sherlock tried not to laugh at the full use of Mycroft's name and he heard Mary and John snort with laughter.

'You know what?' Cassiopeia said. 'Caring might not be an advantage. Sometimes. I know that better than you two idiots because I did- I do care about people. About the ones I love. When you lose someone it hurts like hell but the feeling you get when you don't lose them. It outweighs the bad. It makes you happy.'

'I am happy.' Sherlock and Mycroft said quickly.

'You're content,' Cassiopeia said, 'not happy. You two seem to forget that there is no other person in this _world_ that knows you better than I do.'

And in that moment, Sherlock Holmes and Mycroft Holmes, both realised how much they had missed the youngest Holmes.

But the game had just began.

***TGU***

 **A/N: This chapter was going to end in many ways but I like this. Now, I'm Australian and so if you see things that don't sound British please let me know :) I love to improve and I sincerely hope that you like this chapter and will stick with this story :) Please review :)**


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